In liquid cooled nuclear reactors fuel elements may be advantageously monitored by the use of ultrasonic transducers. Such monitoring includes imaging the structure of fuel pins to ascertain their location and type. In addition, it is possible to detect the presence of minute ruptures in the outer cladding surface of fuel elements. Imaging and detection is achieved by transmitting ultrasonic waves into the reactor and analyzing the reflected waves. A typical transducer for this purpose may consist of a laminate including one or more slabs of a piezoelectric crystal contacted by electrodes and protected by a lens from the hostile environment in which it must operate. The function of the lens is to isolate the crystal from the surrounding liquid coolant and thereby prevent its degradation. Thus, the transfer of acoustic energy to or from the crystal occurs by way of the lens and hence the lens and crystal must be in intimate contact with each other.
Heretofore, one technique of joining the lens and crystal has been by means of compression. The technique consists of compressing a crystal between two lenses, using mechanical means such as bolts or springs. As the pressure is necessarily high (500-1000 psi) to assure proper coupling of the acoustic waves into or out of the crystal, a relatively thick lens is required. If the lens is too thin it tends to deform, thereby stressing the crystal and resulting in a crystal fracture. A salient disadvantage of the thick lens necessitated by the compression technique is that the lens degrades the high performance and fidelity of the transducer.
Another technique heretofore known is to braze the lens to the crystal. This allows the selection of a lens of a thickness considerably less than that used in the above described compression technique. However, one of the problems associated with brazing is that the coefficients of thermal expansion of the lens and crystal in their mating surfaces must match in order to prevent irregular fracturing of the crystal. Alternatively, the lens material must be sufficiently weak to allow it to flex. A further complicating factor is the fact that the coefficient of thermal expansion of a piezoelectric crystal generally varies with its crystallographic direction. Hence, to achieve a perfect match of the thermal coefficients of the crystal and lens is virtually impossible.
At temperatures below 200.degree. C. it is possible to match the coefficients of thermal expansion of certain crystals with certain lens materials. For instance, the piezoelectric crystal material known as PZT-5 has a symmetric thermal expansion coefficient. Thus, it has been successfully bonded to stainless steel having an approximately matching coefficient of expansion. The use of PZT-5 material is, however, precluded at higher temperatures because the stress resulting from the thermal mismatch at these temperatures becomes too great for the crystal to withstand. Moreover, the Curie temperature of PZT-5 material is 250.degree. C., which serves as an absolute limit on its useful range.